1. Field of the Invention
The present invention relates to proton exchange membranes, and in particular relates to composition and formation methods to improve dimensional stability of the membrane.
2. Description of the Related Art
Nafion® (sulfonated tetrafluorethylene copolymer) is a conductive polymer developed by Dupont in the 1960's. This compound is also called ionomer due to its ionic property that comes from the sulfonatic acid grafted on the terminal of the polytetrafluorethylene bone chain. Nafion has excellent thermal stability and mechanical properties, thereby being of interest for researchers, researching proton exchange membrane fuel cells (PEMFC).
The proton of the sulfonatic acid in Nafion can transfer from one solfonic acid to another for proton exchange. On the contrary, the electron and cation of the sulfonatic acid in Nafion cannot transfer from one solfonic acid to another for proton exchange.
Although Nafion has many advantages, its size changes substantially after being heated. U.S. Pat. No. 4,983,690 discloses a Nafion and bismaleimide blend to improve the mechanical properties of films made thereby, however, its dimension still changes after heated.
Accordingly, a novel method for improving physical properties of a Nafion film is called for.